1. Match the meter to your film
Set the metering app to the ISO you intend to use for the roll. For a first test, use the film’s box speed: ISO 400 film means ISO 400 in the meter. Changing this setting does not change the sensitivity of film already loaded in the camera.
Choose the setting that matters to the photograph first. A moving subject might make shutter speed your priority; a portrait might make aperture your priority. Check that the suggested values exist on your camera and lens.
2. Check what an iPhone light meter sees
Obtura measures reflected light through the phone camera. Bright and dark areas can influence that reading. A portrait against a bright window needs a more deliberate measurement than an evenly lit wall. ILFORD explains the distinction between reflected and incident metering in its light-meter guide.
Compare a reading of the overall scene with one concentrated on your subject. Keep the phone in the same light and avoid casting your own shadow across the area you measure. If the readings differ, decide which part of the photograph needs the most attention rather than averaging numbers blindly.
3. Transfer an equivalent exposure
Imagine a meter reading of f/8 at 1/125 second, ISO 400. The following pairs give approximately the same exposure. This is a calculation example, not a reading for every daylight scene.
| Aperture | Shutter speed | Trade-off |
|---|---|---|
| f/5.6 | 1/250 s | Faster shutter, less depth of field |
| f/8 | 1/125 s | Starting pair |
| f/11 | 1/60 s | More depth of field, greater motion-blur risk |
Opening the aperture by one stop lets in roughly twice as much light; halving the exposure time balances it. Set both camera dials, then check them again before releasing the shutter.
4. Keep a short test record
For your first roll, note the frame number, lighting, meter reading and settings actually used. Compare the developed results with those notes. If readings differ consistently from a trusted meter, compare both on the same evenly lit target before changing calibration. One difficult backlit frame is not enough evidence.
For long exposures or lens filters, consult the film and filter manufacturers’ instructions. Do not assume a phone app has already accounted for those corrections.
Try the workflow in Obtura
Obtura offers reflected spot and average metering, camera profiles and a shot journal. Its preview is simulated, so use your negatives and test notes to judge results. The public beta requires iOS 26 or later and TestFlight. Metering works without Apple Intelligence.